The decision by Elizabeth Filkin, the parliamentary standards commissioner, not to re-apply for her job follows a "dirty tricks" campaign in parliament and Whitehall involving cabinet ministers, MPs and Whitehall press officers.

Mrs Filkin was hounded by senior ministers and leading figures in the Conservative party almost from the time she took up the job in 1998 after a series of damning reports upset members of the government and prominent Tories.

Her letter reveals she has on file a list of named Whitehall press officers who were instructed by members of the cabinet to brief lobby journalists against her when her inquiries started to unsettle leading figures.

At one stage it was suggested that she complained to Sir Richard Wilson, the cabinet secretary, about ministers' behaviour - a move she decided to reject on the grounds that it would generate more heat than light on the question.

The Guardian understands that two members of Tony Blair's last cabinet - Peter Mandelson, when he was trade secretary, and John Reid, when he was Scottish secretary - are among those ac cused. They were both under investigation for breaching Commons rules and the MPs' code of conduct.

A third former minister, Keith Vaz, still under investigation for undeclared links with the controversial Hinduja brothers , also employed - despite a denial to the Guardian - a whispering campaign to undermine her.

Mr Mandelson is understood to have done this when he was facing a full scale investigation into why he had failed to declare a £373,000 loan from Labour millionaire Geoffrey Robinson to buy his Notting Hill home. Mr Mandelson is said to have been incensed about her finding that he had broken the code by omitting the existence of the loan when he applied for a mortgage on the property with the Britannia Building Society.

Mr Reid - accused of bullying by Mrs Filkin after he had used taxpayers' money to employ his son and another researcher on Scottish Labour party business - is also alleged to have used Whitehall to discredit her.

The Guardian has learned that in Mr Vaz's case no officials were used as intermediaries. Instead he made private phone calls to lobby journalists spreading stories of alleged conflict of interest involving Mrs Filkin's husband, Michael Honey, while Mr Vaz was under investigation by her.

Other ministers and prominent Tories bypassed Mrs Filkin by going straight to the former speaker, Betty Boothroyd, to complain about what they called her overzealous investigations.

These included the former prime minister, John Major, and Mr Mandelson. Their complaints led to Lady Boothroyd reporting inaccurate criticism of cases handled by Mrs Filkin in her biography.

These actions have led Mrs Filkin to be particularly angry about the lack of independence in her job. MPs put out the idea that she could not wait to investigate them for the most trivial reasons. In fact, nine of 10 cases brought to her were dismissed without an investigation because the basic accusation did not even stand up.

The other big complaint brought by MPs is that she was always looking at " tit for tat" investigations - brought by Tories against Labour and vice-versa. This led to a number of small scale rulings against prominent figures like former Tory leader William Hague for not registering his use of Jeffrey Archer's gym and Robert Sheldon, the Labour chairman of the standards and privileges committee, for failing to declare a shareholding.

It even caused a row over Mr Major's lucrative speaking engagement consultancy - where she was blamed after acquitting him for seeking the best advice. In many of these cases she was being used for party political purposes by MPs who then turned on her as a scapegoat for bringing the Commons into disrepute.

Her other big disclosure is the failure of the House of Commons commission - the shadowy body of six MPs chaired by the speaker - to back her up with proper resources to do her job. Over the last three years every review has said she needed more staff to do her job, while the commission has only given her a minute increase in part-time staff. The commission's defence is that it is facing huge demands from the rest of parliament for more staff.

Last night, even as her decision to go next February became public, the rumour mill against her was still going strong. It is now being said that she is leaving because she has signed a book deal to reveal "all".

This rumour is based on the fact that she has been seen taking extensive notes at meetings of the standards and privileges committee - part of her job. There is no book deal.